In order to produce hydrocarbons economically, a reasonably accurate estimation of hydrocarbon volume and moveability has to be performed. The conventional resistivity interpretation method becomes unreliable if the formation water resistivity is unknown or if it is very high. Additionally, the formation factor and the cementation exponent of the formations can vary with depth, resulting in an inaccurate water saturation evaluation.
The measurement of dielectric constant (or dielectric permittivity) of formations surrounding a borehole is known to provide useful information about the formations. The dielectric constant of the different materials of earth formations vary widely (for example, roughly 2.2 for oil, 7.5 for limestone, and 80 for water), so measurement of dielectric properties can be a useful means of formation evaluation. Logging tools for this purpose have been proposed and/or developed over the years.
A logging device which measures formation dielectric constant is disclosed in the U.S. Pat. No. 3,944,910. The logging device includes a transmitter and spaced receivers mounted in a pad that is urged against the borehole wall. Microwave electromagnetic energy is transmitted into the formations, and energy which has propagated through the formations is received at the receiving antennas. The phase shift and attenuation of the energy propagating in the formations is determined from the receiver output signals. The dielectric constant and, if desired, the conductivity of the formations, can then be obtained from the phase and attenuation measurements. Measurements are typically, although not necessarily, made on the formation invaded zone. Two transmitters are generally used in a borehole compensated array to minimize the effect of borehole rugosity, tool tilt, and dissimilarities in the transmitters, receivers, and their circuits. (See, for example, U.S. Pat. No. 3,849,721.)
The antennas shown in U.S. Pat. No. 3,944,910 are slot antennas, each having a probe that extends across the slot in a direction parallel to the longitudinal direction of the borehole. This configuration has become known as a “broadside” array. The U.S. Pat. No. 4,704,581 describes a logging device of similar type, but wherein the slot antennas have probes that extend in a direction perpendicular to the longitudinal direction of the borehole. This configuration has become known as an “endfire” array. These two different configurations have orthogonal magnetic moments and some other differences. The endfire array exhibits a deeper depth of investigation and is less affected by tool standoff (e.g. from mudcake or poor pad contact) than the broadside array. On the other hand, the broadside array exhibits a stronger signal characteristic than the endfire array.
A logging device which utilizes teachings of the above-referenced U.S. Pat. Nos. 3,944,910 and 4,704,581 is the electromagnetic propagation tool (“EPT”—mark of Schlumberger). A so-called adaptable EPT (“ADEPT—mark of Schlumberger”) can provide either broadside operation or endfire operation during a given logging run, depending on conditions. The ADEPT logging tool has two changeable pads, one containing a broadside antenna array and the other an endfire antenna array.
In the U.S. Pat. No. 5,434,507 there is disclosed a logging device that includes a two-dimensional array of slot antennas which can provide two-dimensional quantitative dielectric and conductivity images of formations in the region surrounding the borehole. The '507 Patent states that by obtaining measurements over a two-dimensional array, and using then known log interpretation techniques, one can obtain, for example, two-dimensional maps of water-filled porosity. The tool of the '507 Patent is also stated to be useful in permitting obtainment of azimuthal measurements of dielectric constant and conductivity, for example when characterizing dipping or fractured beds and other heterogeneities such as vugs or localized washouts. Reference can also be made to U.S. Pat. Nos. 5,243,290 and 5,345,179.
Dielectric logging tools, such as those described, measure effective formation permittivity and conductivity. The formations consist of the rock matrix and the pore fluids (usually hydrocarbon and water). In order to deduce the volumetric fraction of water from the effective permittivity, one has to know the relationship (the so-called mixing law) between the properties of the constituents and their mixture. Among several existing dielectric mixing laws, the CRIM (Complex Refractive Index Method) has been one of the most widely utilized formulas in the area of petrophysics (see e.g. Calvert, T. J., Rau N. R., “Electromagnetic Propagation, A New Dimension In Looging”, SPE 6542, 1977.) Also, variations of CRIM were developed, such as the CTA (Complex Time Average) method (Cheruvier E., Suau J., “Application of Microwave Dielectric Measurements in Various Logging Environments”, 1986, SPWLA 27th Annual Logging Symposium). Mixing laws require knowledge of the matrix and fluid complex permittivity at downhole conditions. Often it is difficult to predict these values accurately due to unknown matrix mineral composition and the resistivity of the formation water.
The conventional resistivity interpretation method becomes unreliable if the formation water resistivity is unknown or if it is very high. Additionally, the cementation and saturation exponents of the fundamental Archie relationship (which relates measured resistivity to water resistivity and water saturation) can vary with depth and/or radius, thereby complicating the accurate evaluation of the formation water saturation. Also, the texture of formations can have a major impact on hydrocarbon productibility and can affect downhole measurements. This characteristic currently cannot be determined in-situ and usually requires an expensive and time-consuming coring program.
It is among the objects of the present invention to devise and/or improve methods for accurately estimating the cementation and saturation exponents from downhole measurements, hence allowing for more accurate determination of water saturation and hydrocarbon saturation. It is also among the objects of the present invention to provide techniques that take rock texture into account in determination of characteristics relating to formation permittivity and conductivity and in the computation of water saturation and other formation characteristics.